The invention relates to an arrangement for coupling in of microwave energy into a reaction chamber.
In the coating of substrates, especially of calotte-shaped or cup-shaped substrates, the latter are exposed, in a reaction chamber, to the plasma of a reaction gas stimulated by means of microwaves. The quality of the coating depends, inter alia, on the constancy of the introduced microwave energy. If variations are encountered with respect to the radiated-in microwave energy, the coatings of the successively treated calottes exhibit differing qualities. The components responsible for coupling the microwave energy into the reaction chamber exert a substantial influence on the constancy of the introduced microwave energy.
DE 3,905,303 Al describes an apparatus for the production of a plasma by means of microwaves wherein a discharge tube is arranged in the coaxial line, this tube passing over into a reaction chamber. The coaxial waveguide and the reaction chamber form, together with the discharge tube, a resonator for the microwave frequency utilized. Microwave loss is reduced by the provision of a metallic throttle or of a metallic attachment at the end plate mounted to the coaxial waveguide. A similar arrangement is disclosed in DE 4,028,525.
U.S. Pat. No. 4,886,346 describes an arrangement of microwave components capable of producing a rotationally symmetrical microwave field (E.sub.01 microwave field) in order to generate thereby a rotationally symmetrical plasma in a glass recipient. For tuning the microwaves to be introduced, the coupling-in arrangement exhibits two tuning elements requiring, for an electrically stable operation, an exact tuning in the 1/10 mm range. Changes in length in this order of magnitude can easily arise if the temperature of the resonator is not accurately stabilized.
This is very expensive, with the relatively large dimensions of the coupling-in device. Furthermore, the frequency of the magnetron as the microwave source, preferably utilized for high power, changes with the temperature, the output, and the age. This likewise results in detuning so that the microwave energy radiated into the reaction chamber does not correspond to the desired values.
A so-called "Surfaguide" structure has been disclosed in J. Physique 43, 1982, L 71 consisting, in part, of the same components, in principle, as in U.S. Pat. No. 4,866,346; by means of this structure, a rotationally symmetrical microwave field is produced for the buildup of a plasma. This arrangement likewise requires two tuning elements which must be very accurately adjusted and must retain this setting within very narrow tolerances.
U.S. Pat. No. 4,971,651 discloses a process and apparatus for coupling in microwaves wherein absorbing or scattering components are introduced into the zone of the microwave radiation in such a way that electromagnetic fields which, without this measure, would locally raise the plasma density, are attenuated or eliminated by scattering. In this way, the distribution of the intensity of the electromagnetic field in the plasma and thus the result of the microwave treatment are to be made uniform. According to U.S. Pat. No. 4,971,651, the absorber or the scattering component must exhibit an exactly defined shape and must be arranged at exactly defined locations within the microwaveguide so that they can fulfill their objectives. Shape and locations must be determined in preliminary tests. If, for example, the configuration of the microwaveguide and the discharge vessel are changed, then location and shape of the absorber and/or of the scattering component must be newly determined for ensuring uniformity. However, U.S. Pat. No. 4,971,651 yields no hint as to how a microwave coupling into a plasma must be structured so that such coupling is extensively tolerant with respect to changes of geometrical dimensions and outputs of the coupling arrangement.